1. Field of the Invention
The present invention relates generally to the field of electrical circuit interrupting devices adapted to complete and interrupt electrical current carrying paths between a source of electrical power and a load. More particularly, the invention relates to a novel technique for rapidly interrupting an electrical circuit and dissipating energy in a circuit interrupter upon interruption of a current carrying path.
2. Description of the Related Art
A great number of applications exist for circuit interrupting devices which selectively complete and interrupt current carrying paths between a source of electrical power and a load. In most conventional devices of this type, such as circuit breakers, a movable member carries a contact and is biased into a normal operating position against a stationary member which carries a similar contact. A current carrying path is thereby defined between the movable and stationary members. Such devices may be configured as single-phase structures, or may include several parallel mechanisms, such as for use in three-phase circuits.
Actuating assemblies in circuit interrupters have been developed to provide for extremely rapid circuit interruption in response to overload conditions, over current conditions, heating, and other interrupt-triggering events. A variety of such triggering mechanisms are known. For example, in conventional circuit breakers, bi-metallic structures may be employed in conjunction with toggling mechanisms to rapidly displace the movable contacts from the stationary contacts upon sufficient differential heating between the bi-metallic members. Electromechanical operator structures are also known which may initiate displacement of a movable contact member upon the application of sufficient current to the operator. These may also be used in conjunction with rapid-response mechanical structures such as toggle mechanisms, to increase the rapidity of the interrupter response.
In such circuit interrupters, a general goal is to interrupt at current close to zero as rapidly as possible. Certain conventional structures have made use of natural zero crossings in the input power source to effectively interrupt the current through the interrupter device. However, the total let-through energy in such devices may be entirely unacceptable in many applications and can lead to excessive heating or failure of the device or damage to devices coupled downstream from the interrupter in a power distribution circuit. Other techniques have been devised which force the current through the interrupter to a zero level more rapidly. In one known device, for example, a light-weight conductive spanner is displaced extremely rapidly under the influence of an electromagnetic field generated by a core and winding arrangement. The rapid displacement of the spanner causes significant investment in the expanding arcs and effectively extinguishes the arcs through the intermediary of a stack of conductive splitter plates. A device of this type is described in U.S. Pat. No. 5,587,861, issued on Dec. 24, 1996 to Wieloch et al.
While currently known devices are generally successful at interrupting current upon demand, further improvement is still needed. For example, in devices that do not depend upon a natural zero crossing in the incoming power, back-EMF is generally relied upon to extinguish the arcs generated upon opening, which, themselves, define a transient current carrying path. The provision of spaced-apart splitter plates establishes a portion of this transient current carrying path and represents resistance to flow of the transient current, producing needed back-EMF. However, depending upon the level of power applied to the device, such sources of back-EMF may be insufficient to provide sufficient resistance to current flow to limit the let-through energy to desired levels. In particular, splitter plates, as one of the sources of back-EMF, may fail at higher voltage levels (current tending to shunt around the plates, for example), imposing a limitation to the back-EMF achievable by conventional structures. As a result, depending upon the nature of the event triggering the circuit interruption, the excessive let through energy can degrade or even render inoperative the interrupter device.
There is a need, therefore, for an improved circuit interrupting device which can provide efficient current carrying capabilities during normal operation, and which can rapidly interrupt current carrying paths, while limiting let through energy to reduced levels by virtue of rapid arc extinction. There is a particular need for a new device structure which is economical to manufacture and can be packaged in various sizes and ratings.
The invention provides a novel technique for interrupting an electrical circuit and for dissipating energy in a circuit interrupter designed to respond to these needs. The technique may be employed in a wide variety of circuit interrupting devices, such as circuit breakers, motor controllers, switch gear, and so forth. Moreover, the technique may be incorporated with various interrupter structures, such as interrupters employing a light-weight spanner displaced under the influence of an electromagnetic field generated by a core, as well as various other triggering mechanisms which initiate circuit interruption.
In accordance with the technique, a transient current carrying path includes at least one variable or controllable resistance element. The element establishes a preferred current path during an initial phase of circuit interruption to cause current flow through the transient current carrying path and thereby to interrupt flow through a normal or main path through the interrupter. The element then changes a conductive state to enhance the energy-dissipating capabilities of the transient current carrying path. In a preferred configuration, a variable resistance structure is positioned adjacent to incoming and outgoing conductors, and is in a relatively conductive state during the initial phase of circuit interruption. Current through arcs during this initial phase of interruption is conveyed into the transient current carrying path by virtue of the resistance of the element. A rapid change in the resistive state of the element then ensues, contributing to rapid interruption of the transient current by contributing additionally to the back-EMF through the device. The change in resistive state may be a function of heating by the transient current. The novel structure may be employed in both single and multi-phase circuit interrupters. The elements which establish the transient current carrying path, and which change their resistive state, may be static components, such as a polymer in which a dispersion of conductive material is doped, or what may be referred to as positive temperature coefficient (PTC) materials. The transient or alternative current carrying path may include a series of splitter plates separated by air gaps and electrically in series with the variable resistance element. The transient current carrying path may thus present an essentially open circuit during normal operation of the device, and may comprise only mechanically static elements electrically in parallel with the normal current path through the interrupter.